Your due date, also called the estimated date of delivery, is 280 days from the first day of your last menstrual period (LMP), if you have regular monthly periods.
Most women deliver in the two weeks before or after their due date. About 4 percent of pregnant women deliver on their due date. Very few expectant moms know exactly when they conceived, however.
Even if you only had sex once during your fertile period, you wouldn't necessarily conceive right then.
It's possible that the sperm hung out in your fallopian tube for a few days until your egg came along, at which point it fertilized the egg and your baby was conceived. So without knowing the day of conception, anyone can determine a due date.
Calculating your due date isn’t an exact science. Very few women actually deliver on their due date, so, while it’s important to have an idea of when your baby will be born, try not to get too attached to the exact date.
Naegele’s rule involves a simple calculation: add seven days to the first day of your LMP and then subtract three months.